This invention relates to hot melt adhesive formulations based on styrene-diene block copolymers. More particularly, the invention relates to specific adhesive formulations for use in the production of pressure sensitive, peelable labels.
Pressure sensitive adhesive (xe2x80x9cPSAxe2x80x9d) adhesive labels are highly desirable for a wide variety of purposes. For instance, pages of peelable label stock are available which can be fit into standard printers to print address labels, etc. The preaddressed labels are then peeled off and put on envelopes. There are a wide variety of uses for such labels and many varieties, sizes, and types are available commercially.
One method for producing sheets, etc. of PSA peelable labels involves providing a base sheet which is generally coated with a release coating. Release coatings are materials from which the label can be peeled away such that the adhesive is not removed from the label and retaining its adhesive properties. Thus, the label then has the capability of being adhered to another substrate.
A separate sheet of label material which can be printed or written upon is formed and a label adhesive is coated on the side of the sheet opposite to the side upon which it is intended to be printed or written upon. The label sheet is then adhered to the release coating sheet by placing the adhesive surface in contact with the release coating surface.
In one method of producing such labels, the product, which is the two sheets adhered together, is called roll label stock and the operation for producing the finished product is called label die cutting. The basic method is shown in FIG. 1. The roll label stock 1 is fed through a rotary die 2 which has several cutters 3 which produce the desired shape of the cut out 4 in the roll label stock 1. The roll label stock 1 then continues on to the stripping roller 5 which removes the matrix flagging 6 from the roll stock 1, leaving behind the die cut labels 7.
Obviously, it is important that the rotary die 2 be able to cut through the adhesive and the paper of the top layer of roll label stock 1. The adhesive must be strong enough, however, to leave the label behind on the release-coating layer 8 of roll label stock 1. Styrene-diene block copolymers have been found to be useful in such applications as the base material for the adhesive because of their desirable adhesive strength and also because of their relatively high tensile strength. However, the tensile strength of the styrene-diene block copolymers used heretofore sometimes is disadvantageous because the rotary die 2 will not completely cut through the top layer of roll label stock 1 so the labels 9 continue to stick to the cut out matrix flagging 6.
It would be highly advantageous if an adhesive formulation could be found which maintains the tack and adhesive performance of the aforementioned adhesive formulations but which has lower cohesive strength as evidenced by lower elongation at break and lower tensile at break so that the labels could be completely cut from the matrix and would remain on the release coating layer. The present invention provides such an adhesive formulation.
This invention is an adhesive formulation for die cuttable labels which comprises:
a. 100 parts by weight of a styrene-isoprene-styrene block copolymer having an overall number average molecular weight of 50,000 to 450,000, a polystyrene content of about 10 to about 25 percent weight, and a coupling efficiency of less than 70 percent,
b. from 5 to 30 parts by weight per 100 parts of block copolymer of polystyrene having a number average molecular weight of at least 50,000,
c. from 120 to 200 parts by weight per 100 parts of block copolymer of a tackifying resin, and
d. from 5 to 50 parts by weight per 100 parts of block copolymer of oil.